As vehicular anti-vibration structures, there have been known, from Japanese Utility Model Application Laid-Open Publication No. SHO-61-38341 etc., anti-vibration bushings connecting vehicle-body-side members and suspension arms.
FIGS. 16A and 16B are explanatory of the conventional suspension bushing disclosed in the above-mentioned No. SHO-61-38341 publication. More specifically, FIG. 16A is a sectional view of an anti-vibration bushing constructed as the suspension bushing. This anti-vibration bushing 200 includes an inner cylindrical member 201 fixed to the body-side member, an outer cylindrical member 202 fixed to a rear suspension arm 200A and surrounding the inner cylindrical member 201, and a resilient member 203 joining together the inner and outer cylindrical members 201 and 202.
The resilient member 203 has a pair of hollow sections 204 opposed to each other with the inner cylindrical member 201 interposed therebetween. Each of the hollow sections 204 has a bulge 206 bulging radially outward, substantially in a shape of a mountain ridge, from its surface closer to the inner cylindrical member 201 toward the outer cylindrical member 202, and a peak (outer end) portion of the bulge 206 is abutted against its surface closer to the outer cylindrical member 202.
FIG. 16B is a graph showing relationship between load applied to the outer cylindrical member 202 via the rear suspension arm 200A in a direction of arrow P and amount of displacement of the outer cylindrical member 202 caused by the applied load through resilient compression of the resilient member 203. In the graph, the vertical axis represents the applied load, while the horizontal axis represents the displacement amount of the outer cylindrical member 202 through the compression of the resilient member 203. Specifically, the graph shows a nonlinear spring characteristic of the anti-vibration bushing 200, in accordance with which the characteristic curve presents a small inclination (i.e., the spring constant is small) while the displacement amount is small and the inclination (i.e., the spring constant) progressively becomes greater as the displacement amount increases.
With the nonlinear spring characteristic as shown in FIG. 16B, the spring constant remains relatively small while the displacement amount is in a medium value range. Thus, when, for example, the human driver or driver rapidly depresses and releases an accelerator pedal for rapid acceleration, in the vehicle employing the suspension bushing of FIG. 16A, so that a great load is applied in the direction of arrow P from a drive road wheel via the rear suspension arm 200A, forward/rearward vibration of the vehicle sometimes can not be suppressed effectively due to a poor damping performance of the anti-vibration bushing 200.
Namely, the anti-vibration bushing 200 does not function or contribute as a spring while the displacement amount of the outer cylindrical member 202 is in a small value range. If the characteristic curve inclination is increased in the example of FIG. 16B in order to allow the anti-vibration bushing 200 to contribute as a spring, the inclination would become excessively great in a great value range of the displacement amount so that the bushing 200 can not effectively absorb shocks. Further, in the suspension bushing of the Japanese patent, relationship between load applied to the outer cylindrical member 202 in an opposite direction to the arrow P direction and amount of displacement of the outer cylindrical member 202 caused by the applied load through resilient compression of the resilient member 203 is set to present a load vs. displacement characteristic curve that is symmetrical to that of FIG. 16B about the origin point of the graph of FIG. 16B; that is, the load vs. displacement characteristic curve is not differentiated in accordance with the direction of the applied load.